I did use the Boson's for the CCSA NGX. However they were very poor quality and I did not feel as they helped much for the exam. They have been constantly updating the test so maybe by now they are much better quality as the quality did improve a bit with each update. It is difficult for any NGX exam as there are not many practice tests out there. I passed the ccsa ngx and used mainly the Syngress Configuring Checkpoint NGX book and all the pdf documents I could find on Checkpoint's web site. I would stay away from testking or actual tests as they are frequently wrong on their answers and they do not help you learn given how they get the questions.
Best advice I can give is to read all the pdf documents on checkpoints web site:
http://www.checkpoint.com/support/te.../docs_r60.html These documents are very informative and were a big help for the CCSA NGX. And they are free so you really can't go wrong. Good luck on the exam!